1. Field of the Invention
The present invention relates to a thermal protector suited for the protection of a motor which drives a hermetic compressor or the like of an air conditioner or an electric refrigerator.
2. Description of the Related Art
A thermal protector is mounted to a motor which drives a hermetic compressor or the like of an air conditioner or an electric refrigerator to protect the motor in an overload condition. The thermal protector of this type is disclosed by, for example, Japanese Unexamined Patent Application Publication No. 61-227631, which thermal protector will now be described with reference to FIG. 10A. A depressed portion 212a is formed at a base 212 of the thermal protector. A tray type bimetal 240 is housed in the depressed portion 212a. A heater 250 is provided below the tray type bimetal 240 and a movable contact plate 230 is provided above the bimetal 240. The movable contact plate 230 has one end secured to a fixed pin 252 and has a free end to which a movable contact 232 which comes in contact with a fixed contact 234 is mounted. The heater 250 is connected in series with a motor. The heater 250 generates heat by a current flowing through the motor and thereby heats the bimetal 240. FIG. 10A shows the bimetal 240 which carries out a downward snap action in a steady state. If the motor is overloaded and abnormal current flows through the motor, the bimetal 240 heated by the heater 250 carries out an upward snap action as shown in FIG. 10C. The bimetal 240 pushes the movable contact plate 230 up and separates the movable contact 232 from the fixed contact 234, thereby terminating current flow through the motor.
In the thermal protector with the above-stated structure, the bimetal 240 is only housed in the depressed portion 212a and not secured. This facilitates assembling the bimetal 240 into the thermal protector, but on the other hand, makes it difficult to keep a long stroke S (the traveling distance of the bimetal shown in FIG. 10C) of the action point of the bimetal 240 (the contact point of the bimetal 240 with the movable contact plate 230). As a result, a snap action tends to be delayed.
By "delay in snap actions" we mean as follows. The state of the bimetal bent to the greatest extent at ordinary temperature shown in FIG. 10(A) is changed to a state in which the bimetal is bent to a smaller extent when the bimetal is abnormally heated and then to a state in which the bimetal is bent to the smallest extent (in a concave state) shown in FIG. 10B. From this smallest bent state, the bimetal is reversed to a convex state. It is now necessary to separate the movable contact 232 from the fixed contact 234 while the snap action is being carried out. When the bending of the bimetal is at the smallest state, just before the snap action as shown in FIG. 10B, the movable contact 232 separates slightly from the fixed contact 234 just before the snap action is carried out, whereby an electric arc may be produced therebetween. The delay in snap action is also caused by bringing the movable contact 232 into contact with the fixed contact 234 just before the bimetal carries out a snap action or is reversed from a convex state to a concave state shown in FIG. 10(D). In order to prevent the delay of snap action during operation and recovery, the thermal protector structure as stated above only requires adjusting the position of the movable contact plate 230.